The present invention relates to methods and apparatus for forming extrudable materials such as plasticized powder batches or pastes into complex structures, such as honeycombs, by the process of extrusion. More particularly, the invention relates to improvements in extrusion apparatus and extrusion methods that enable the production of cellular structures such as extruded honeycombs from extrudates such as plasticized ceramic or metal powder batches at higher extrusion rates and lower extruder pressures.
The manufacture of inorganic honeycomb structures from plasticized powder batches comprising inorganic powders dispersed in appropriate binders is well known. U.S. Pat. Nos. 3,790,654, 3,885,977, and 3,905,743 describe extrusion dies, processes, and compositions for such manufacture, while U.S. Pat. Nos. 4,992,233 and 5,011,529 describe honeycombs of similar cellular structure extruded from batches incorporating metal powders.
While continually evolving, current commercial honeycomb extrusion die designs do not depart fundamentally from the die designs shown in these early patents, being mostly fabricated by the machining of solid metal blocks or billets. To make such a die, multiple apertures are first drilled into one face of a steel billet to form a feedhole array into which a plasticized batch material to be extruded can be supplied at high pressure. A discharge face for the die is thereafter formed by cutting a criss-crossing array of finely machined discharge slots into the billet face opposite the drilled inlet face, the slots being cut to a depth intersecting the ends of the feed holes extending from the inlet face. Thus plasticized batch material delivered through the feedholes into the intersecting discharge slots is continuously shaped by and discharged from the slots as the interconnecting wall-and-channel structure of an extruded honeycomb.
A number of machining techniques have been adapted for the shaping of metal billets into honeycomb extrusion dies. For softer steels, the feedhole array is typically formed by mechanical drilling and the discharge slots by sawing. If the die is to be formed of harder, slower-wearing materials such as stainless steels, electrochemical machining and electrical discharge machining are more widely used. Generally, however, inlet face designs continue to feature feedholes in the shape of linear cylinders of reasonably constant radius and of a diameter and spacing dictated by the slot spacing or so-called honeycomb cell density of the die.
Of constant concern in the manufacture of ceramic honeycombs by extrusion methods are the high pressures required to force plasticized powder batch materials through these dies. Commercial demand for honeycombs with thinner walls and higher cell densities (more channels per unit of honeycomb cross-section) has required dies with smaller and more closely spaced feed holes and discharge slots, and necessitated the use of higher extruder pressures and thus dies of higher strength.
A number of methods for increasing die rigidity and/or reducing die flow impedance have been proposed to address these problems. Die designs such as the compound feed dies taught in U.S. Pat. No. 4,118,456 can increase die strength and rigidity, but the feedhole and slot flow channels in these dies still present planar surfaces oriented largely transversely to the direction of batch flow through the die. These surfaces tend to increase flow impedance and create flow stagnation points within the dies, the latter acting as sources for agglomerate formation that can lead to clogged passages within the dies and/or regions of batch inhomogeneity caused by the preferential collection of batch constituents such as water, plasticizers, and lubricants at the stagnation points.
Smoother flow profiles are provided in laminated dies such as those disclosed in U.S. Pat. No. 6,558,151 and bonded pin dies such as disclosed in published Japanese Patent Application No. 62-236,710, but at a significantly higher die fabrication cost. Also costly are die designs such as disclosed in U.S. Pat. No. 5,066,215 wherein transverse surfaces along the batch flowpath are completely eliminated. In each of these cases the manufacture of even a single extrusion die is complex and expensive, and thus the maintenance of an inventory of such extrusion dies adequate to support an economic ceramic honeycomb manufacturing operation can be cost prohibitive.
In short, currently available extrusion die designs of sustainable cost continue to present significant problems relating to high pressure drop, problems that are aggravated as higher cell density honeycombs requiring more complex dies and higher extrusion pressures are commercially required.